This invention relates to an electrophotography and, more particularly to an electrophotographic apparatus in which a number of copies can be formed from a single electrostatic charge latent image.
In electrophotograpy it has been known to form a copy image by effecting the following successive steps; i.e. a step of forming an invisible electrostatic charge latent image on a member such as a photosensitive drum, an electrostatic charge transfer drum, etc., which can retain the electrostatic charge latent image for a relatively long time period, a step of developing the latent image with electrostatically charged toner particles and a step of transferring a developed toner image onto a toner image receiving member such as a plain paper. For the transferring step there have been proposed several processes. In a corona discharge transfer process, for example, onto a rear surface of the toner image receiving member, i.e. paper is applied a corona charge having a polarity opposite to that of the toner particles. In a bias roller transfer process there is provided a transfer roller composed of an electrically conductive roller with or without a thin insulating layer coated thereon and the paper is fed between the latent image retentive member and the transfer roller with applying a suitable bias voltage thereacross. In the corona discharge transfer process a homogeneous corona charge which is essential for effecting complete transfer could not be applied on the rear surface of the paper unless a corona charge potential is made sufficiently high. On the other hand when a plain paper having a resistance of about 10.sup.10 .about.10.sup.14 .OMEGA.-cm is used, a corona ion stream might be injected onto the latent image retentive member through the paper under the high corona charge potential and as a result the latent image might be deteriorated or destroyed and overdevelopment or fog might occur in a final copy. In the bias roller transfer process a transferring efficiency is very low due to undesired phenomenon that a center part of the toner image on the charge retentive member is remained untransferred in dependence on a contact pressure of the transfer roller, an amount of developed toner particles and a fluidity of particulate toners. If the bias voltage is made as high as to obtain a satisfactory development efficiency, the latent image might be deteriorated as in the case of the corona discharge transfer process. Thus, in general the bias roller transfer process is not suitable for a high speed transfer process.
In electrophotography it has been also known to form a number of duplicated copies from a single electrostatic latent image. In this case the latent image once formed on the charge retentive layer of the transfer drum or photosensitive drum is alternately and repeatedly subjected to the toner development and transfer processes.
If the above explained known corona discharge and bias roller transfer processes are utilized in the electrophotographic apparatus which forms a number of copies from the single electrostatic latent image, it is quite difficult to obtain a number of copies of high image quality. This is due to the fact that the electrostatic image is deteriorated by the corona charge and bias potential and the toner image on the retentive member is also damaged to a great extent by discharge which might be produced when the paper leaves the charge retentive member. Thus the quality of the copy images is extremely low and overdevelopment might appear as a dot form.
The inventor has proposed an improved electrophotographic process which can obviate the above mentioned drawbacks and a number of copies can be formed at a high speed from a single latent image without deteriorating it. In this process bias voltages are applied across the latent image storing member and the paper and across the charge storing member and an electrically conductive screen or grid arranged between the storing member and the corona charger, respectively and the paper is charged with corona ions passing through the grid.
FIG. 1 is a schematic view of an electrophotographic apparatus which operates under the above improved printing process. In this embodiment as an electrostatic charge retentive member use is made of a transfer drum 1 composed of an electrically conductive roller 2 arranged rotatably in the direction shown by an arrow and an electrostatic charge retaining layer 3 coated on the surface of roller 2. As the charge retentive layer 3 use may be made of insulating material having a high dielectric constant such as a glass film having a high lead content, an organic resin such as acryl resin, polyurethane resin, epoxy resin, etc. On the transfer member 1 an electrostatic latent image 5 corresponding to an image of a document (not shown) is formed by a latent image forming device 4 in accordance with a known method such as a Transfer of Electro-Static Image (TEST) method, corona ion stream modulation method with using a photosensitive screen, a multistylus method commonly used in the field of facsimile. The latent image 5 may be positive or negative in polarity. In the present case the latent image has a negative polarity.
Then the electrostatic latent image 5 is developed with toner particles by a developing device 6 in accordance with a known development process such as a cascade development process and a magnetic brush development process. In this manner a visual toner image 7 is formed on the transfer drum 1. Next this toner image is transferred onto a toner image receiving member such as a plain paper.
At the transfer position there are provided a pair of transfer rollers 8A and 8B in the close vicinity of or in contact with the transfer member 1, an electrically conductive grid 9 of about 100 to 200 mesh arranged in opposite to the transfer drum 1 between said transfer rollers 8A, 8B and a corona charging device 10 arranged in opposite to the transfer member 1 with respect to the screen 9 and connected to a corona voltage supply source 12. During the transfer process a paper 11 is fed with being in contact with the transfer drum 1 with a slight pressure by means of the transfer rollers 8A, 8B and the corona charger 10 projects through the screen 9 corona ions having a polarity which is same as that of the charge forming the latent image, but is opposite to that of the toner particles forming the visual toner image. There are further arranged voltage sources 13 and 14 between the conductive drum 2 and grid 9 and between the conductive drum 2 and transfer rollers 8A, 8B, respectively so as to apply suitable transfer bias voltages thereacross, respectively.
By means of such a transfer process the toner image 7 can be effectively transferred onto the paper 11 at high speed without deteriorating the latent image on the transfer drum 1. Then the paper 11 having the toner image transferred thereto is separated from the drum 1 with the aid of a scraping claw 15. The toner image thus transferred is fixed in a known fixing process and a final duplicated copy is formed. The latent image 5 on the drum 1 is effectively remained and thus by carrying out alternately and repeatedly the development and transfer processes a number of copies such as 200 to 250 copies can be obtained from the single latent image 5 once formed on the transfer drum 1.
After a given number of copies have been formed the remaining latent image and residual toner particles are removed by means of a known cleaning device 16 so as to prepare for a next printing operation.
In the electrophotographic apparatus shown in FIG. 1 if the toner image receiving member, i.e. paper 11 is alway existent at the transfer position and the transfer corona charge voltage and bias voltage are in good condition, a number of copies can be obtained from a single latent image. But, in practice the toner image receiving member is not always existent at the transfer position. For instance in case of using sheet papers successive papers are fed into the transfer position in an intermittent manner and during intervals between successive papers the transfer rollers 8A, 8B and the corona charger 10 face directly the transfer drum 1. This is also the case in which a roll paper and a cutter are used. During such intervals the transfer corona charge and bias voltage are directly applied to the charge retentive layer 3 of the transfer drum 1 and as a result the retentive layer 3 is charged with the same polarity as that of the latent image 5. Thus in a next development process toner particles might adhere to undesired portions of the transfer drum 1, which have been newly charged in the previous transfer process, as well as to the electrostatic latent iamge. In a next transfer process the toners on the undesired portions might be transferred to a front or rear edge of a next coming paper 11 and thus the paper might be soiled or stained. Further the toners on the undesired portions of the transfer drum 1 might adhere to the transfer rollers 8A and 8B and as a result the rear surface of the paper 11 becomes soiled. Moreover since the bias voltage is applied to the charge retentive layer 3 directly by the transfer rollers 8A, 8B there might be produced breakdown in the retentive layer 3 and if the retentive layer 3 is made of photosensitive material, its life time might be extremely shortened.
FIG. 2 shows a situation in which the front edge of the paper 11 is just inserted into the transfer position. Since the transfer bias voltage is applied before the paper 11 enters in a transfer region T, the charge retentive layer 3 is charged by means of the transfer corona charger 10 and transfer rollers 8A, 8B with the same polarity as that of the latent image 5. In case of forming a single copy the residual charge and toners on the retentive layer 3 are removed by the cleaning device 16 (FIG. 1) and thus there is not problem. But in case of printing a number of copies from the single latent image with repreating alternately the development and transfer processes it is not suitable to operate the cleaning device 16 during the printing operation, because it affects the latent image 5 very much. Therefore in case of the multiple printing the charge deposited on the undesired portions of the charge retentive layer 3 is developed with toners and the toners are liable to be transferred onto the front and rear edges of the paper 11, because usually it is quite difficult to registrate accurately the paper with respect to the transfer drum 1.
Further when the toners adhered to that portions of the transfer drum 1 which are free from the latent image 5 come under the transfer range T, these toners 17 are subjected to corona ions as shown in FIG. 3 and are charged with neutral or negative polarity. Since the adhesive force of these toners is weak, they are liable to be transferred to the downstream roller 8B or to spread in a space and thus the front and rear surfaces of the paper 11 might be soiled to a great extent.